The present invention relates to induction furnaces, and more particularly to induction furnace monitoring equipment.
Coreless induction furnaces have long been used for the heating and melting of metals. The more common metals heated are alloys of iron, steel, copper, brass and aluminum; however, other highly conductive materials can be melted using the induction principle.
An induction furnace comprises a refractory lining to contain the melt bath of metal to be heated, a power coil surrounding the refractory lining, and a plurality of magnetic yokes for concentrating the magnetic field established by the coil.
The power coil includes a plurality of turns and is typically fabricated of copper or aluminum. The coil carries large electric current which establishes a magnetic field which in turn induces electric currents, called eddy currents, in the molten metal bath. The power coils can have a variety of configurations. Some furnaces have single coils with typically 10 to 35 turns. Other furnaces have more than one coil, and yet other furnaces have concurrently wound coils. The power coils are connected to power sources of various voltages and frequencies, which can be either single phase or multi-phase.
The power coil is surrounded by a plurality of generally vertically oriented silicon steel columns called magnetic yokes, which are electrically grounded. The yokes support the power coil and provide a magnetic path for the alternating magnetic flux field which is created by the alternating electric current in the power coil.
For safety reasons, the melt bath is electrically grounded through rods which extend through the bottom of the refractory lining. The ground rods are fabricated of highly conductive, high temperature metal, which insures that the melt bath is at ground potential at all times.
Coreless induction furnaces have many problems causing unsafe conditions, costly downtime, lengthy maintenance inspections, and expensive repairs. First, when a conduction path occurs between a power coil and a ground, known in the art as a ground fault, an electric detection system senses the ground fault condition and trips, or turns off, the power supply. A ground fault condition can be caused by an object external to (i.e., outside) the coil touching both the coil and a grounded member of the furnace. Often, such an external ground fault is caused by a metal part wedged between a coil turn and one of the grounded magnetic yokes. An internal ground fault occurs when the melt bath penetrates the refractory lining and engages the coil. Because the bath is grounded, the coil is then also grounded through the melt bath. When a ground fault is detected, the fault must be located and cleared before the molten bath solidifies, typically 3 to 10 hours after power is lost. If the fault is not timely found, the melt bath must be drained. Currently, any ground fault, either external or internal, must be located by visually inspecting the coil. Visual inspection is inherently slow and consequently results in expensive losses in terms of production loss, heat energy loss, and possible refractory damage. Typical times required to find ground faults in medium to large furnaces (8 to 80 tons) range from 2 to 48 hours. Further, visual inspection does not readily reveal ground faults located behind magnetic yokes or those due to melt bath penetration through the insulative refractory lining.
Second, when a weak spot in the refractory occurs, the molten bath can penetrate the refractory and leak out past the power coil. This condition, referred to as runout, is an expensive and dangerous condition. Possibly both coil and structural damage can result, requiring excessive repair cost and time. An internal ground fault typically precedes a run-out. However, visual inspection of the coil when a ground fault is detected will not readily reveal whether the fault is external or internal. Consequently, the operator upon detecting a ground fault knows that a certain possibility exists that the fault is internal and might lead to a run-out. Upon detecting the ground fault, the operator may either drain the melt bath to insure prevention of run-out, or retain the melt bath and run a risk of subsequent run-out. Premature draining of the melt bath results in excessive losses, as outlined above, while retaining the melt bath when an internal ground fault does indeed exist seriously increases the risk of run-out damage.
Third, refractory linings wear as melt batches are made. Refractory materials have defects and inconsistencies which make linings wear at different rates. As the linings wear, the refractory wall thickness decreases, occasionally resulting in a metal run-out through a weakened or thin portion of the wall. Presently, linings are visually inspected when the furnace is drained, typically only every few weeks. Because the linings typically last only from three to ten weeks, this visual inspection results in only a crude approximation of lining wear between inspections. Often, a refractory liner does not wear consistently and can become excessively thin soon after a visual inspection indicates that the liner is safe.
Fourth, when ground faults occur, the power system is tripped off quickly. Consequently, electrical instruments and meters, which might give clues to the nature of the trip problem, go dead when power is lost. This lost information is difficult, if not impossible, to recapture once the trip occurs.
Fifth, some furnaces have more than one power coil which are tied together through bus switches when a ground fault occurs. Currently, the only method for determining which coil is grounded is by isolating each coil and testing that coil with an ohm meter. This requires excessive time resulting in excessive losses, as outlined above. All previous detection systems used with cables for fault detection are not applicable to coils because of the mutual flux linkage between coil turns. Cables have distributed impedances, but coils have uneven impedances along its length.